From a certain point of view, they are huge.
I once read (*) that the full gamma burst from a thermonuclear explosion takes several seconds. (Whereas from a fission bomb, it's mostly over in a fraction of a second; the bigger-yield fusion bombs create a lot of temporary unstable shit that gives off more gamma rays as it decays over several seconds.) This led to me developing a nuclear war survival trick, which I will now share with everyone on Slashdot, even though I haven't tested (**) it yet:
If there's a sudden blinding flash in the sky, quickly try to estimate: does it look like a big one? If so, then dive for cover, preferably behind something big and solid, like a boulder or something like that. HTH.
(*) Wish I could cite a reference, but I'm lazy.
(**) If my trick is no good or based on misunderstood physics, you can make fun of me after the next nuclear war.